Elimination of SF6 From Transmission System Equipment A Thesis Resubmitted To The University of Manchester For The Degree of PhD In The Faculty of Engineering and Physical Sciences 2013 by Xiaolei Cai School of Electrical and Electronic Engineering Contents List of Tables ................................................................................................................................ IV Table of Figures ........................................................................................................................... VI List of Abbreviations ..................................................................................................................... XI Nomenclature .............................................................................................................................. XII Declaration ……………………………………………………………………………………………..XIII Copyright …………………………………………………………………………………………….XIV Abstract ……………………………………………………………………………………………….1 Chapter 1 Introduction ................................................................................................................ 2 1.1 Background of Project ................................................................................................... 2 1.2 Use of SF6 in Gas Insulated Substations ...................................................................... 6 1.3 Project Objectives and Thesis Structure ....................................................................... 9 Chapter 2 SF6 Gas and Equipment Design ............................................................................. 11 2.1 Properties of SF6 Gas ................................................................................................. 11 2.2 Electrical Performance of SF6 Gas ............................................................................. 13 2.3 The Use of SF6 in Circuit Breakers ............................................................................. 14 2.3.1 Benefits of SF6 Switchgear in Comparison to Oil or Air ...................................... 16 2.4 SF6 Use in Gas Insulated Substations ........................................................................ 17 2.5 Benchmark Busbar Design for Use in Future Studies ................................................ 19 2.6 Conclusion of SF6 Properties and Gas Insulated Equipment Study Model ................ 19 Chapter 3 Potential Liquid and Gaseous Replacements for SF6 as An insulation Medium .... 21 3.1 Electrical Breakdown in Liquid Dielectrics .................................................................. 21 3.2 Recent Developments in Oil Insulation ....................................................................... 22 3.2.1 Oil Insulation Performance .................................................................................. 22 3.2.2 Illustration of Possible Size of Oil Insulated Substation and Key Barriers to Its Use …………………………………………………………………………………………23 3.2.3 Summary of Oil Insulation ................................................................................... 25 3.3 Electrical Breakdown in Gas Dielectrics ..................................................................... 26 3.3.1 Gas Ionization and Attachment Processes ......................................................... 26 3.3.2 Townsend Breakdown in Electronegative Gas ................................................... 27 3.4 Review of Gas Alternatives to SF6 .............................................................................. 28 3.4.1 Gas Mixtures with Small Volume of SF6 ............................................................. 28 I 3.4.2 Electronegative Gas - CF3I.................................................................................. 30 3.4.3 High Pressure Air, N2 or N2O ............................................................................... 34 3.5 Conclusion of Liquid and Gaseous Replacement for SF6 ........................................... 35 Chapter 4 Estimate of Size of Alternative Gas Insulated Substations ..................................... 37 4.1.1 Benchmark Design for Comparison and Discussion ........................................... 38 4.1.2 Fixed Size of GIS Enclosure with Changing Gas Pressure ................................ 46 4.1.3 Fixed Gas Pressure with Changing Busbar/outer Shell Dimensions .................. 50 4.1.4 Impact of Reduced Basic Insulation Level .......................................................... 51 4.2 Analysis of the Use of Coatings Combined with Gas Insulating ................................. 54 4.2.1 Review of Voltage Capability of Insulation Coatings ........................................... 55 4.2.2 Assessment of Use of Coatings around Conductors .......................................... 56 4.2.3 Review of Current Carrying Capability of Coated Conductor ............................. 57 4.3 Conclusion of Alternative Gas Usage ......................................................................... 60 Chapter 5 Evaluation of Foam As a Replacement for SF6 as an Insulation Medium .............. 62 5.1 Electrical Breakdown in Solid Insulation ..................................................................... 62 5.2 Solid Polyurethane Foam Insulation ........................................................................... 64 5.2.1 Electrical Breakdown in Polyurethane Foam ...................................................... 64 5.2.2 Review of Use of Foam in HV Applications ........................................................ 66 5.3 Experimental Study of Polyurethane Foam ................................................................ 69 5.3.1 Production of Polyurethane Foam ....................................................................... 69 5.3.2 Measurement of Relative Permittivity.................................................................. 70 5.3.3 Measurement of Thermal Conductivity ............................................................... 72 5.3.4 Analysis of Void Distribution ................................................................................ 73 5.3.5 Measurements of Partial Discharge Inception Voltages ..................................... 76 5.3.6 Measurement of AC Voltage Strength of Filled Samples ................................... 78 5.3.7 Measurement of Lightning Impulse Strength ...................................................... 81 5.3.8 Estimation of Size of Busbar Dimensions ........................................................... 83 5.3.9 Summary of Foam Insulation .............................................................................. 84 5.4 Conclusions Including a Comparison of Alternative Insulation Materials ................... 85 Chapter 6 Potential Replacements for SF6 as An Interruption Material ................................... 87 6.1 Physics of Circuit Breaker Switching .......................................................................... 87 6.1.1 Arc in Circuit Breaker .......................................................................................... 88 6.1.2 Current Chopping ................................................................................................ 88 6.1.3 Switching Transients in Network ......................................................................... 90 6.2 Comparison of Existing Interruption Systems ............................................................. 90 6.3 Review of Alternative Circuit Breaker Types ............................................................... 93 6.3.1 Oil Circuit Breakers ............................................................................................. 93 II 6.3.2 Air Blast Circuit Breakers .................................................................................... 95 6.4 Gas Circuit Breakers ................................................................................................... 97 6.4.1 Background ......................................................................................................... 97 6.4.2 Effect of PTFE Ablation on Puffer Circuit Breaker .............................................. 98 6.4.3 SF6-Free Gas Circuit Breaker ........................................................................... 100 6.5 Conclusion of Existing Non-SF6 Circuit Breaker ....................................................... 102 Chapter 7 Vacuum Circuit Breaker ........................................................................................ 103 7.1 Introduction to VCB Technology ............................................................................... 103 7.2 Recent Development of Vacuum Circuit Breaker ..................................................... 104 7.2.1 Contact Material ................................................................................................ 104 7.2.2 Contact Structure .............................................................................................. 106 7.2.3 Control of X-ray Emission ................................................................................. 109 7.2.4 Review of Transmission Voltage Vacuum Circuit Breakers .............................. 110 7.3 Development of Multiple Interrupters Based VCBs................................................... 111 7.4 Transient Voltage Simulations Using a Vacuum Circuit Breaker Model ................... 113 7.4.1 Transient Recovery Voltage .............................................................................. 114 7.4.2 Prestrike, Restrike and Reignition Phenomena ...............................................